U.S. officials said it's not the first time China has placed J-11 and J-7 fighter jets on Woody Island, part of the Paracel island chain in the hotly disputed region, after expanding the runway there in 2014.
"The specifics of a recent deployment of fighter aircraft to Woody Island would be less an issue than the signal it sends of how far out of step China's actions are with the aspirations of the region," a Pentagon spokesman said in a statement.
The placement of missiles on the island, which appear to be permanently placed in concrete, is of greater concern, according to the officials.
China controls the Paracel chain, but Taiwan and Vietnam also claim it; Brunei, Malaysia and the Philippines are also embroiled in disputes with China over regional islands and their surrounding waters.
Lt Col Yang Zhiwei, Taiwan's Defense Ministry spokesperson, also confirmed the deployment and called on all parties to the dispute to avoid escalating tensions.
The move comes as Chinese Foreign Minister Wang Yi met with
U.S. Secretary of State John Kerry in Washington on Tuesday.
China deployed surface-to-air missiles
on Woody Island, according to Taiwan and U.S. officials last week, in a move that has alarmed the country's Asian neighbors.
Chinese state media
said defenses had been in place on the island for years, and denied it was militarizing the island.
Satellite images taken on February 14 appeared to show several missile batteries and support vehicles, according to ImageSat International
, which took the images.
Taiwan's Defense Ministry said it had confirmed that surface-to-air missiles had been deployed. U.S. officials, who spoke on condition of anonymity, told CNN that commercial satellite imagery showed the deployment of missile batteries.
The news ratcheted up tensions in the volatile region, already home to messy territorial disputes, with a senior Japanese Cabinet member labeling China's actions unacceptable.
China's Defense Ministry has said defense facilities had existed on the islands for years, according to the government-run Global Times. It wasn't clear whether the report was referring to the surface-to-air missiles identified by Taiwan officials, which satellite imagery suggested had been deployed this month.
China has occupied Woody Island for 50 years.
China's Ministry of Foreign Affairs spokesman Hong Lei told reporters last week that the "deployment of defense facilities in our own territory is appropriate and reasonable."
"It's aimed at improving our national defense capabilities and has nothing to do with so-called militarization," he told a press briefing.
Greg Poling, head of the Asia Maritime Transparency Initiative at the Washington-based CSIS, told CNN that China was also developing radar capabilities
on islands in the South China Sea, allowing more effective control of the sea and airspace.